1. Field of the Invention
This invention relates to processes for preparing aqueous polymer emulsions and more particularly, to processes for preparing such emulsions from aqueous monomer-polymer dispersions. This invention also relates to aqueous polymer emulsions.
2. Description of the Prior Art
Aqueous polymer emulsions are generally produced by two methods. One method is the polymerization of monomers, e.g., polyvinyl acetate/acrylate copolymers from vinyl acetate and acrylic monomers. A second method is a post-dispersion technique, often by brute force, of bulk polymers in a liquid state, i.e., crystalline and semi-crystalline polymers in a fluid molten state, amorphous polymers in a liquid state, or polymers in organic solvent solution.
The first method is restricted to monomers which undergo free radical polymerization or copolymerization to form the desired polymer composition, structure, molecular weight, etc. Condensation polymers, cellulosics and many hydrocarbon resins are excluded from this method of forming an aqueous emulsion. Furthermore, end-use performance advantages afforded by polymer blends and readily gained in solvent-based systems are difficult and often impossible to duplicate with aqueous emulsion systems, since blends of such emulsions consist of heterogeneous particles. Addition of one or more monomers to a preformed latex "seed" of a different polymer composition either requires a lengthy diffusion time or affords a heterogeneous core/shell structure; in either case, increased particle size generally is inevitable.
The second method primarily is useful for oils, resins generally well above their glass transition temperatures under practical dispersion conditions, and for polymer solutions in organic solvents. Polymers with high glass transition temperatures, or with poor flow above this temperature, such as cellulose acetate butyrate, and polymers which decompose before significant flow, such as nitrocellulose, must be dissolved in an organic solvent prior to such aqueous dispersion. Polymer concentration in the organic solvent may be limited by viscosity, especially if polymer molecular weight is appreciable. Presence of solvent also seriously limits polymer concentration in the final emulsion, unless the solvent is carefully removed e.g., through evaporation. The latter step may be lengthy and costly.